Add a new built-in theme witiko/diagrams

[Witiko]

This PR makes the following changes:

• Add version v1 of the new built-in theme witiko/diagrams, which aliases the earlier theme witiko/dot.
• Soft-deprecate the theme name witiko/dot in favor of witiko/diagrams@v1.

This PR also makes the following changes:

• Introduce built-in example themes witiko/example/foo and witiko/example/bar.
  These themes are designed for debugging purposes and to serve as examples in code demonstrations. When loaded, they emit a warning but produce no other side effects or functional changes.
• Explictly specify the line length limit for explcheck in the CI workflow.

Continues #531.